Hybrid ARQ Protocol for Multi-Antenna Multicasting Using a Common Feedback Channel

Wireless multicasting (also called common information broadcasting) is a technique where a common information message is transmitted to multiple users. This is typically accomplished by having the basestation broadcast out a signal representing this message. In this paper, we consider a multicasting scheme where the basestation transmits the multicasting signal without any a-priori knowledge of the users´ channel state information. In this set-up, a hybrid automatic repeat request (ARQ) control is employed to improve the reliability of multicasting communication. In most hybrid ARQ set-ups, each user is usually allocated a dedicated feedback channel to tell the basestation if the previously transmitted signal was correctly decoded. However, dedicated feedback channels waste significant uplink resources, especially when the number of users is large. To mitigate this problem, we consider a negative acknowledgement (NACK) based hybrid ARQ control where the failed users are allowed to transmit the NACK signal (which is assumed to be the same for all users) through a common channel while the other users remain silent. We consider the effect of feedback error and multicasting signal decoding error on our hybrid ARQ system performance as the number of users grows large. Specifically, it is shown that the throughput performance of the proposed hybrid ARQ scheme using repetition retransmission is not severely degraded relative to the throughput performance of the noiseless feedback case. In addition, we show that the performance improvement obtained by replacing repetition encoding with incremental redundancy becomes insignificant for the large number of users asymptote. On the other hand, it is shown that the improvement provided by incremental redundancy is significant when some of the users are allowed to fail to decode the message.